The present invention relates to nuclear boiling water reactors (BWRs) that utilize natural circulation and more particularly to providing for an improved steam separator-dryer in such reactor designs.
Existing large BWRs are of the forced-circulation type. In BWRs undergoing power generation operations, reactor coolant, initially in the form of sub-cooled liquid (e.g. water), is circulated by main coolant recirculation devices (e.g. jet pumps or mixed-flow motor-driven pumps) around a path a portion of which is comprised of a core lower plenum region (located at the bottommost section of the reactor), thence through the nuclear core and into a core upper plenum in communication with the core. Flow exiting the core upper plenum then passes through standpipes that lead to an assembly of steam separators. The reactor coolant exiting the nuclear core and passing into the core upper plenum is a two-phase mixture of steam and water, the proportion of which varies depending upon such factors as the power output from the fuel bundles, the amount of sub-cooling present in the coolant entering the fuel bundle, and the amount of flow through the bundles.
Mechanical steam separation is generally utilized to accomplish the separation of the steam from the steam/water mixture exiting the core supplied to the turbine. This separation must be effected because if steam moisture contents are too high in the turbine steam flow, accelerated erosion can occur in the steam lines and on first-stage turbine blades and the efficiency of the turbine is reduced. The steam separator assembly typically consists of a domed or flat-head base on top of which is welded an array of standpipes with a three-stage steam separator, for example, located at the top of each standpipe. One function of the standpipes is to provide a stand-off separation of the larger-diameter steam separators, which are generally arranged in a particularly tightly-compacted arrangement in which external diameters of adjacent separators are nearly touching with each other, so that separated liquid coolant discharged at the bottom of the separator has a more "open" flow path outwardly from the reactor longitudinal axis and out to a downcomer annulus region which lies at the inboard periphery to the reactor pressure vessel.
The steam separator assembly rests on the top flange of the core shroud and forms the cover of the core discharge plenum ("core upper plenum") region. In each separator, the steam/water mixture rising through the standpipes (the "standpipe region") impinges upon vanes which give the mixture a spin, thus enabling a vortex wherein the centrifugal forces separate the water from the steam in each of three stages. Steam leaves the separators at the top of this assembly and passes into a wet steam plenum below a dryer assembly. The separated water exits from the lower end of each stage of each separator and enters a pool or "downcomer region" that surrounds the standpipes to join the downcomer flow. From each dryer in the dryer assembly exits substantially moisture-free steam that is used to drive a turbine generator which, in turn, is coupled electrically to a grid.
The steam separator assemblies heretofore known in the art are typically bolted to the core shroud flange by long hold-down bolts, or the separator assembly together with the dryer assembly are held down onto the core shroud flange by contact from the reactor head when the latter is assembled to the reactor vessel. However, as shown, for example, in U.S. Pat. Nos. 4,912,733 and 3,902,876, the steam separator assembly and the dryer assembly are not integrated; but, instead, are present in the reactor pressure vessel (RPV) housing the reactor as discrete components. As a consequence, during refueling of the reactor core, separate storage pools are needed to store the respective assemblies until the refueling operations are completed. The need for those separate storage pools generally is seen as adding to the expense of a nuclear reactor system.
Moreover, considering that full-scale testing of separator-dryer combinations can only be accomplished in operating reactors and that, pursuant to current practice, there must be a minimum distance between the top of the separator and the bottom of the dryer, it may be seen that not all combinations of conventional separators and dryers may be experimentally tested. Consequently, there remains a need for an improved and modular steam separator and dryer assembly adapted for testing in existing experimental facilities.